This is meant to be used in Mythic Sire Denathrius P3 to basically carry the players with the debuffs with as little thought required. This can be fairly confusing when you're first progressing the fight and might help people progress P3 a bit quicker than they would otherwise.
This weakaura will keep track of every Fatal Finesse cast and will send a corresponding message to the player depending on the cast number and whether they are in the mirror phase or in the outside phase.
It will automatically detect whether you are in the mirror phase or not, so this was able to be compacted into one aura.
The messages can be customized to whatever you want them to be, but I've placed the ones that my guild used for our kill in by default.
Please Note: This was one of the first custom weakauras I've actually spent some time on and I did this in order to start learning about how to make weakauras as I couldn't find one on wago at the time. I've already found out that I can make this a lot better with TSU/events but I haven't gone through the process of re-writing it yet. Also, I am In the middle of making the reminder text pop up about 5s before the Fatal Finesse comes out to give you a bit more thinking time. Although I haven't had the chance to test it at all, you can find it here and use at your own risk.
You can see the setup of the P3 markers here as reference for the default messages:
Adjust how you see fit for your P3 setup